ios - 如何在SwiftUI中汇总过滤后的核心数据？

Question

如何在SwiftUI应用中求和(使用谓词)过滤后的核心数据值？

我有1个名为 NPTransaction 的实体。它有5个属性，我想对来自名为 value(Integer 64)的属性的值求和。基本上，它将汇总所选月份的收入值(现在在示例代码中将其设置为当前Date()，但稍后将其设置为整个当月)。

我发现了有关Swift的类似问题，但无法使此代码在我的应用中正常工作。
链接到相关问题:How to sum the numbers(Int16) of stored core data - Swift 3

根据链接中的代码，我现在有以下代码:

import SwiftUI

struct DashboardView: View {
    @Environment(\.managedObjectContext) var managedObjectContext
    @State var selectedDate = Date()

   // This code in private var below produces many errors like:
   // Value of type 'AppDelegate' has no member 'managedObjectContext'
   // Use of undeclared type 'NSEntityDescription'
   // Use of undeclared type 'NSFetchRequest'
   // - so I assume that this linked question may be outdated or I should take different approach using SwiftUI
    private var income: Int64 {
        let context = (UIApplication.shared.delegate as! AppDelegate).managedObjectContext
        let entityDesc: NSEntityDescription = NSEntityDescription.entity(forEntityName: "NPTransaction", in: context)!
        let request: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest()
        request.entity = entityDesc

        request.predicate = NSPredicate(format: "date == %@", selectedDate)
        let records = try! context.fetch(request)

        try! context.fetch(request) as! [NSManagedObject]
        let monthlyIncome = result.reduce(0) { $0 + ($1.value(forKey: "value") as? Int64 ?? 0) }
        return monthlyIncome
    }

    var body: some View {
        NavigationView {
            VStack {                    
               Text("Monthly income:")
               Text("\(income) USD")
                }

// more code...

我只是在学习Swift和SwiftUI，所以也许有更好的和完全不同的方法来解决此问题？

Answer 1

我在这个问题中发现了类似的问题:
Sum of (double) fetch request field
我对代码做了一些修改，并提出了以下解决方案:

import SwiftUI
import CoreData

struct DashboardView: View {
    @Environment(\.managedObjectContext) var managedObjectContext
    // FetchRequest with predicate set to "after now" 
    @FetchRequest(entity: NPTransaction.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \NPTransaction.value, ascending: false)], predicate: NSPredicate(format: "date > %@", Date() as NSDate)) var fetchRequest: FetchedResults<NPTransaction>

    // sum results using reduce
    var sum: Int64 {
        fetchRequest.reduce(0) { $0 + $1.value }
    }

    var body: some View {
        NavigationView {
            VStack(alignment: .leading) {

                HStack {
                    VStack(alignment: .leading) {
                        Text("sum")
                        Text("\(sum)")
                            .font(.largeTitle)

                    }
                    Spacer()
                }
                .padding()

                Spacer()
            }.navigationBarTitle("Title")
        }
    }

}

struct DashboardView_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        return DashboardView().environment(\.managedObjectContext, context)
    }
}